1. Field of the Invention
The present relates to a cleaning unit that performs cleaning by removing a toner remaining on the surface of an image carrier after a toner image is transferred onto paper. The present invention also relates to an image forming apparatus including the cleaning unit.
2. Description of Related Art
In an electrophotographic image forming apparatus, transferring onto paper a toner image formed on the surface of an image carrier or a photosensitive drum is achieved by a method such as transferring a toner image by bringing the paper into direct contact with the photosensitive drum, or first performing primary transfer of the toner image onto an intermediate transfer body formed of another drum or belt and then performing secondary transfer of the toner image onto the paper. Of these methods, the latter transfer method employing the intermediate transfer body is widely used for color printing where a plurality of different colors are used. In this case, toners of the plurality of colors are primarily transferred in a sequential manner onto the intermediate transfer body to form a color toner image thereon, and then a final color toner image is secondarily transferred onto paper at the end.
With such a transfer method, a small amount of toner may remain on the surface of the photosensitive drum even after a toner image is transferred onto paper or the intermediate transfer body. This toner remaining on the surface of the photosensitive drum disturbs the following formation of a new image, thus requiring cleaning. Well-known cleaning methods employed for this purpose includes: transferring a remaining toner onto a rotary member, such as a roller or a rotary brush, for collection by pressing the rotary member against the surface of the photosensitive drum; scraping off a remaining toner with a blade being brought into contact with the surface of the photosensitive drum; and a combination of these methods.
As for an image forming apparatus employing a toner, downsizing of the toner diameter has been pursued in recent years with the objective of forming clear, high-quality images. As a conventional toner manufacturing method, a pulverizing method is used in which toner particles are produced by blowing off a heated plastic material with a strong air pressure and hitting the material against the wall to pulverize it. However, downsizing the toner diameter by use of the pulverizing method results in poor yield and thus an increase in toner manufacturing costs.
Thus, in place of the pulverizing method, a polymerization method has become increasingly widespread as an alternative toner manufacturing method. The polymerization method is a method of producing toner particles by binding substances, sources of a toner, through chemical reaction. In comparison to the conventional pulverizing method, the polymerization method consumes smaller energy, thus attracting attention as a toner manufacturing method that is environmentally friendly. This polymerization method is capable of producing toners of the same shape and size, thus achieving formation of even clearer, higher-quality images.
However, a toner manufactured by the polymerization method suffers from great difficulties in its cleaning, adversely affected by its small diameter and its shape closer to a spherical shape. That is, a toner manufactured by the polymerization method is formed into a shape such that its surface as a whole is hardly caught. This makes it difficult to remove the toner by catching it with the rotary member or the blade. Cleaning methods have been proposed which are capable of achieving a satisfactory cleaning performance by use of an amorphous toner in addition to a spherical toner manufactured by a polymerization method during cleaning of the spherical toner. Japanese Patent Application Laid-open No. H8-254873 discloses one example of such cleaning methods.
With the method of cleaning performed by an image forming apparatus as described in Japanese Patent Application Laid-open No. H8-254873, before development of an image portion with spherical toners of different colors, a development region is formed with an amorphous toner on the non-image area on the leading end side of a photosensitive drum in the surface movement direction thereof so that the amorphous toner is interposed between the blade and a spherical toner upon cleaning, thereby making it easier to clean the spherical toner. However, with this method, an amorphous toner is developed on the non-image area in each development process achieved with each of the spherical toners of different colors, thereby resulting in waste of an unnecessary toner that will never be transferred onto paper, which leads to higher running costs and a possibility of adverse effect from the viewpoint of natural resource saving.